Blowby gasses are combustion gasses that leak from an engine cylinder past piston rings into the crankcase volume. To prevent the blowby gasses from exiting to the atmosphere, positive crankcase ventilation (PCV) systems transfer blowby gasses from an engine crankcase volume to the engine air intake system, where the blowby gasses are mixed with fresh air and are combusted in the engine cylinders. The blowby gasses include unburned fuel, combustion byproducts, and water vapor. The blowby gasses mix with oil mist in the crankcase. An air/oil separator is sometimes used in the PCV system to separate oil from the blowby gasses en route to the air intake system. The engine includes a PCV drain disposed in fluid communication with the air/oil separator to drain the oil that is separated from blowby gasses into the oil pan. The PCV drain may be formed and/or defined by a cylinder head and/or engine block, depending upon the location of the air/oil separator.
The outlet of the PCV drain should remain submerged in oil within the oil pan. If the outlet of the PCV drain becomes exposed, suction from the PCV system may draw the gasses in the oil pan, including oil mist, in a reverse direction up through the PCV drain, through the air/oil separator, and into the air intake system.